1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device. More particularly, the present invention relates to a method of manufacturing an alignment mark.
2. Description of the Related Art
The photolithography process is a technique used to replicate the required pattern into the surface of the wafer, and it is one of the major processes in semiconductor fabrication. The fine circuit diagram becomes more complex as the semiconductor process is continuously improved, so that more and more masks are used in the photolithography process. Typically, in order to accurately replicate the pattern on the mask into the surface of the wafer, the mask must align with the target location in the wafer before the photoresist has been exposed. If misalignment occurs during pattern replication, the replication fails and the wafer is wasted. In the alignment process, the wafer is first localized through a V-shaped notch in the wafer. Alignment marks are formed over the wafer. Then each mark in the mask is aligned with each alignment mark on the wafer. An alignment light shines through each mark of the mask to generate the diffraction light. The diffraction light is received by the alignment sensor to determine whether the mask is aligned with the wafer.
Currently, in the multilevel interconnects process, a dielectric layer is deposited over the substrate after an interconnect layer is formed. Several contact holes or via holes are formed in the dielectric layer and a metal layer is formed over the dielectric layer and fills the contact holes or the via holes. A chemical-mechanical polishing (CMP) is performed to form plugs in the dielectric layer. A subsequent interconnect layer is formed over the dielectric layer. Hence, a multi-layer structure is formed in the integrated circuit product.
However, the step height and the configuration specificity of the alignment marks become indistinct due to the coverage of the multilayer under the alignment marks as the numbers of the deposition layer increases. Therefore, the diffraction light is weak. Additionally, the reflectance and the refraction of each layer in the multilayer structure are different because the materials of each layer in the multi-layer structure are different. Hence, the alignment signal is weak or the noise ratio is large. Therefore, the misalignment occurs.